Question

which part of a bird is evolution of beak nose/tooth/mouth?

Answer 1

It is usually the upper and lower jaw, i.e. the $\boxed{mouth}$.

The mouth is the evolutionary part of the beak forming hard bony structures with light frameworks in the inner side forming upper mandible and lower mandible. Depending upon $\huge \text{adaptive changes}$ the birds developed the beak for various purposes according to their diet, consumption rate and availability of the food in the vicinity.

Beaks have been thought to get evolved from dinosaurs, modern birds adapted due to its radiation of teeth getting evolved into a beak. This was a feature of dinosaur to get the teeth removed by permanent mark and got the evolution to lose all sets of teeth they had in abundance when they get older in later stages forming deformities in bone, this tendency of getting the bone around the jobs made a hint towards the dinosaur to a strong evidence of starting the evolution.

Velociraptor (watch jurassic park series) stand up to 6.85 feet in length, 1.62 feet in height, till 33-35 pounds they can attain weight. Velociraptor are meat-eating carnivorous reptiles who hunted small creatures like, prehistoric birds, reptiles smaller in stature, amphibians, dinosaurs unable to fend themselves, slow-moving dinosaurs, etc. Possessing a tough skull which was long and formed curvatures around the ends forming concave shape (just like a lens) on upper mandible besides convex on the lower mandible. Found predominantly in the period of Cretaceous some 70 or 75 million years ago they were short and fast runners attributing clearly to the slender feet they show.

Evolution just started from here, jurassic birds were another great example to showcase the development of toothed beak into toothless beak. For a specific example, the Jurassic era bird (Archaeopteryx or the prehistoric wing or ancient wing) jaws lined into the rows of teeth which are pointy to tear the meat and chew them for smoother digestion. The diet involved insects (mainly) and sometimes small reptiles and amphibians (frogs and toads). Evolving in late Jurassic period about 150 million to 149 million years ago. Beaks shown in this case showed the hallmark of composition required for a beak in modern birds. Composing mostly of keratin which are also found in fingernails, hairs, feathers, horns of cows, deers, antelopes, etc. Premaxillary bones in birds (to support upper jaw or beak) was not expanded enough in this bird and more fused into the bones rather than showing in prominence.

One more evidence was formation of molecules in genetically altered way making the jaw slight converted to a beak-like structure, this was shown in a gene BMP4 controlling the adaptive functions of the beak, same speciality developed by Archaeopteryx. The gene BMP4 played an important aspect including a role for suppression of teeth rows and increasing the growth of beak by involvement of keratin. Dinosaurs displayed foible flaw and promoted the use of this gene more than the jaw forming structure. This inadequacy and quirk change in the reptilian species meant they already lost their pairs of teeth for a leading keratin beak, shrinking in total stature and size, promoted the sprouting of wings, adopting the use of feathers for using that in flight, improved senses in visual recognition and motor development, shortened tails or complete absence, losing of teeth, cognitive development, and many more characters helping the modern birds hold a spot for their own (kingdom animalia) place in a notion of Class $\bold{Aves}$. Plausible evidences assist us greatly for how the evolutionary and developmental changes are originating from.